1. Field of the Invention
This invention relates to a torque magnetometer and a method of torque magnetometry. The method and magnetometer can be used for measuring extremely small magnetic moments of specimens. Such specimens may for example be ferromagnetic or superconducting specimens. The magnetic moment of a specimen may be studied as a function of temperature or applied magnetic field. Such measurements are relevant to investigations of properties of and processes in magnetic particles and nanostructures.
2. Description of the Related Art
There are a number of methods which can be used to measure a magnetic moment of a specimen. These include SQUID (super conducting quantum interference device) magnetometry, alternating gradient magnetometry and torque magnetometry.
SQUID magnetometers are currently the most sensitive commercially available magnetic moment measuring devices, with a resolution of 10.sup.-10 emu being obtainable. However, SQUID magnetometers have disadvantages. For example, it is not always feasible to make measurements of all three orthogonal components of a magnetic moment because of limitations in current fabrication technology. Further, SQUIDs can only be used in a low temperature environment and are slow to react to changing values of magnetic moment.
The sensitivity of Alternating Gradient Magnetometry is limited by a combination of factors. These include an interfering signal from the diamagnetic moment of a vibrating cantilever on which the specimen is mounted, the sensitivity of a vibration sensor which is used and microphonic noise generated in drive coils used. Microphonic noise is noise caused by the action of Lorentz forces on parts of the equipment. Alternating gradient magnetometry also has the disadvantage that it is not easily adapted to the measurement of more than one component of the magnetic moment.
The principles of torque magnetometry are briefly discussed below.
When a magnetic moment M is exposed to a uniform magnetic field B it experiences a torque T which is defined by equation (1): EQU T=M.times.B (1)
where x represents a vector product, and the magnetic moment, magnetic field and torque are vector quantities.
The torque produced is proportional to the strength of the magnetic field and the magnitude of the magnetic moment. Therefore, if the torque can be measured and the strength of the magnetic field is known, the value of the magnetic moment can be determined. The magnetic moment m can be expressed in terms of components m.sub.x, m.sub.y and m.sub.z. These components being in the x, y and z directions respectively. Similarly, the torque T can be expressed in terms of components T.sub.x T.sub.y and T.sub.z. When making measurements of the torque, the components of the torque can be related to the components of the magnetic moment. Therefore, it is possible to determine the magnitude of each component of the magnetic moment as well as the overall magnitude of the magnetic moment.
In conventional torque magnetometry the torque is generated by the applied magnetising field so the resolution is a function of the applied field magnitude. These magnetometers have the advantage that (isotropic) diamagnetic moments are not measured because such moments align with the applied field and therefore do not generate a torque. However, this means that such magnetometers are only sensitive to magnetic moments which have components which are orthogonal to the applied field since there are no additional magnetic fields applied. The sensitivity of conventional torque magnetometers is limited because no resonant conditions are utilized.
U.S. Pat. No. 5,600,241 discloses a vibrating-reed susceptometer in which a sample whose magnetic properties are to be studied is vibrated, typically mechanically, in the absence of a magnetic field. Once the resonant frequency is known, a dc magnetic field is applied and the effect this has is determined, typically by determining the shift in resonant frequency which occurs.
U.S. Pat. No. 5,001,426 discloses a magnetometer for measuring the magnetic moment of a specimen. This uses an inhomogeneous (spatially varying) magnetic field to exert a force on the magnetic moment of the sample. This is a gradient field magnetometer in which the force (not torque) generated on the sample is used to vibrate the sample at the resonant frequency of the support. The support is then brought to rest by generating an opposing moment using a compensating current loop also located on the sample member. This calls for the use of complicated electronic equipment and means that the device is useful only for relatively large samples--in the range of several millimeters.
U.S. Pat. No. 4,037,149 discloses a multiple mode magnetometer in which both a sample and a detecting coil are driven relative to one another. The drive is generally directly mechanical and no particular use of resonance is made. Once the relative movements are set up by non-magnetic means, an external magnetic field is applied and the resulting effect detected.
A paper by Rossel et al: "Active Microlevers as Miniature Torque Magnetometers" Journal of Applied Physics, Vol. 79, No. 11, 1 Jun. 1996, pages 8166-8173 discloses the use of a mechanically driven cantilever to determine the magnetic moment of a magnetic sample mounted on the lever. The cantilever is driven to resonance by a Bimorph and a uniform magnetic field is then applied. The shift in resonance frequency due to the generated torque is then observed. However it is stated that "the proper correlation of the frequency shift and of the damping to the actual physical parameters of the probe still remain to be clarified." It is assumed therefore that that system could not currently be used to determine a magnetic moment of a sample.
There is a need, not satisfied by the art described above, to provide a magnetic moment measurement system capable of extremely sensitive measurement of magnetic moments which can be employed over a wide temperature range, for example from 4 to 400 K and in high magnetic fields, for example up to 10T.